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Overview
This guide contains best practices for configuring and wiring a BACnet
TP) link for Trane and non-Trane devices.
How a BACnet
A Client Server/Token Passing (MS/TP) link is a type of Local Area Network (LAN) with a set of rules that
control communication traffic. To initiate communications on the network, a device must have the
electronic "token" (a small packet), passed from one device to another around the network. Manager
devices are allowed to have the token, whereas subordinate devices are allowed only to respond to
requests from manager devices. Trane's BACnet
device can send data when it has the token. Each device on the link has a unique physical network address
(rotary switch address). Communication between devices is established within a few seconds of power-
up after all the devices on the link discover other devices.
Each device attempts to locate a successor device at the next highest address by sending a polling
message. If there is no response from the next highest address, the device will poll for one higher address.
It will continue to do so until another device is found that the token can be passed to, or when all 127
possible addresses have been tested.
When a device has the token, it has control of the network and can request data from or write data to other
devices on the link. When it is finished it passes the token to the device at the next address on the link
in sequential order. If the next device has data to send, it does so and then passes the token on. The token
passes from device to device very quickly; it completes the circuit several times per second.
The Trane SerialSpy Protocol Analyzer software looks at all the messages and activity on the BACnet
MS/TP link. The ability to view the token passing is very useful for discovering link problems.
Symptoms of Poor Performance
Symptoms of poor BACnet
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Common Problems
The most frequently encountered problems that can cause poor performance, or non-performance, on a
BACnet
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MS/TP Link Works
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Slow Tracer
Synchrony screen data fill.
Comm loss alarms in the alarm log.
Delayed alarm enunciation between unit controllers and Tracer
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Slow Tracer
TU service tool performance when connected through the SC+ or using TU.
Degradation of building control.
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Slow Tracer
Ensemble screen data fill.
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MS/TP link are:
An open circuit.
A short circuit.
The link has not been installed using a daisy-chain topology.
The wire used for the link does not meet the BACnet
Too many devices on the link.
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Tracer BACnet
terminators are not installed at each end of the link.
Wiring polarity is reversed on the comm link.
When devices are sharing a 24Vac power supply, electrical polarity is not the same for all devices.
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MS/TP links only support manager devices so each
MS/TP performance include, but are not limited to:
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Client Server/Token Passing (MS/
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SC+.
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standard specification.
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BAS-SVX51L-EN
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